The tensioner presses, with a predetermined force, a timing chain or a timing belt that is used, for example, for the engine of an automobile, and maintains the tension constant so as to prevent the chain or belt from becoming elongated or slack.
FIG. 23 shows the condition when a tensioner 100 is mounted to an engine body 200 of an automobile. A pair of cam sprockets 210 and a crank sprocket 220 are arranged inside the engine body 200, and a timing chain 230 is hooked in an endless manner over the sprockets 210 and 220. A chain guide 240 is swingably arranged on the movement path of the timing chain 230, and the timing chain 230 slides on the chain guide 240. A mounting face 250 is formed on the engine body 200, and the tensioner 100 is inserted through a mounting hole 260 that is on the mounting face 250 and the tensioner 100 is fixed to the mounting face 250 by bolts 270. Also, lubricating oil (not shown) is filled inside the engine body 200.
A conventional tensioner includes                a columnar drive member that is movable toward a moving chain,        a case that includes a slide hole through which the drive member is inserted concentrically with the drive member in such a way that the drive member can move freely,        a propelling spring that presses the drive member against the case in the driving direction,        a holder member that is fitted with the drive member in the concave part that is coaxially formed on the tip opening side of the slide hole, is displaced in the axial direction of the drive member, and is provided concentrically with the concave part,        a holder spring that presses the holder member in the driving direction of the drive member,        multiple locking pieces that slide on the inclined cam face and are meshed with multiple locking teeth formed on the outside of the drive member,        a cam-introducing ring that is fitted to the drive member in the slide hole and prevents the multiple locking pieces from becoming detached from the multiple locking teeth, and        an enclosing plate that inserts the drive member and movably encloses the holder spring, the holder member, the locking pieces, and the cam-introducing ring—all of which are sequentially arranged in the concave part of the slide hole—in such a way that the holder spring, the holder member, the locking pieces, and the cam-introducing ring can move freely.        
In this tensioner, if the chain becomes elongated during the operation of the engine, the drive member sequentially moves forward one tooth at a time, thereby maintaining proper chain tension (for example, see Patent Document 1).
In the tensioner described above, the drive member is pressed forward by the propelling spring, and the diameter of the locking pieces is thereby enlarged, as a result of which the locking pieces can move forward while riding over the locking teeth of the drive member. Also, when the drive member moves backward, the locking pieces are pressed against the inclined cam face, and the diameter of the locking pieces is thereby reduced. The locking pieces then mesh with the locking teeth of the drive member, so that the locking pieces are prevented from moving backwards, and are locked in that condition.
When mounted to the engine body, the drive member moves forward to a position whereby proper chain tension is maintained, and the drive member is prevented from returning excessively from that forward position while receiving vibrations from the chain guide. If an excessive load is applied, the holder member moves backward, and the holder spring is flexed so as to keep proper chain tension. Also, if the chain has become elongated due to a long period of use, the drive member appropriately moves the chain guide forward, so that proper chain tension is maintained.    Patent Document 1: Japanese Patent No. 3717473
FIG. 24(a) is a vertical cross-sectional view of a conventional tensioner that has a constitution similar to the tensioner described in the above-specified Patent Document 1. FIG. 24(b) is a side view of that tensioner. FIG. 25(a) shows how the drive member of the tensioner in FIG. 24 is fully locked with the locking pieces. FIG. 25(b) is a cross-sectional view along the line D-D of FIG. 25(a). FIG. 26(a) shows how the diameter of the locking pieces is enlarged when the drive member of FIG. 25 moves forward, and FIG. 26(b) is a cross-sectional view along the line E-E of FIG. 26(a).
In these drawings, number 310 is a drive member; 320 is a locking piece; 330 is a holder member; 340 is a propelling spring; 350 is a pressing spring that presses the locking pieces 320 against the inclined cam face 330a of the holder member 330 so as mesh with the drive member 310, so that the diameter of the locking pieces 320 is reduced; 360 is a holder spring; and 370 is a case.